1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular, a liquid crystal display device including pixel electrodes and drain lines formed in the same layer.
2. Description of the Related Art
A liquid crystal display device called in-plane switching (IPS) or lateral electric field type has a configuration in which pixel electrodes and a common electrode that are provided so as to be opposed to each other through the intermediation of an insulating film are formed on the same flat substrate. In the IPS liquid crystal display device, an electric field having a component parallel to a main surface of the flat substrate is generated between the pixel electrodes and the common electrode so that liquid crystal molecules are driven by the thus generated electric field. With the configuration described above, the IPS liquid crystal display is known as being capable of performing display with a wide viewing angle. In particular, in a so-called C-top type IPS liquid crystal display device including a common electrode formed on the side closest to a liquid crystal layer, a transparent electrode having a plurality of slits formed therethrough is formed on the liquid crystal side of flat-plate like pixel electrodes made of a transparent conductive film through the intermediation of an insulating film. With the configuration described above, an electrode having a linear pattern (comb-like electrode) overlapping the flat-plate like pixel electrodes is formed.
As an example of the liquid crystal display device having the configuration described above, there is known a liquid crystal display device described in Japanese Patent Application Laid-open No. 2008-276172. In the liquid crystal display device described in Japanese Patent Application Laid-open No. 2008-276172, each of slits of a common electrode has a linear continuous portion between adjacent pixels. Thus, the liquid crystal display device, in which the continuous portions are symmetric with respect to a boundary of the pixels so as to reduce viewing-angle dependence, is disclosed.
Further, Japanese Patent Application Laid-open No. 5-265045 discloses a liquid crystal display device in which two gate lines are arranged for one line in a scanning direction, and one drain line is provided for each set of two pixels on each display line.
In the conventional liquid crystal display devices, a plurality of drain lines and a plurality of gate lines are formed on a glass substrate so as to cross each other. A video signal is input to the drain lines, whereas a scanning signal is input to the gate lines. A region surrounded by adjacent drain lines and adjacent gate lines is a pixel region. For each pixel region, a pixel electrode is formed.
The C-top type IPS liquid crystal display device is also known, which includes drain lines and pixel electrodes formed in the same layer. In the liquid crystal display device having the configuration described above, gate electrodes and gate lines are formed on a surface of a glass substrate. An insulating film functioning as a gate insulating film of a thin-film transistor is formed so as to cover the upper surface of the glass substrate. On a surface of the insulating film, the drain lines and the pixel electrodes are formed. The drain lines and the pixel electrodes are covered with an interlayer insulating film made of an inorganic material, which is formed so as to cover the upper surface of the substrate. A transparent conductive film serving as a common electrode is formed on a surface of the interlayer insulating film.
When the technology described in Japanese Patent Application Laid-open No. 2008-276172 is applied to the liquid crystal display device having the configuration described above, the drain lines and the common electrode overlap each other through the intermediation of the insulating film made of the inorganic material, having an extremely smaller thickness than that of the interlayer insulating film made of an organic material, between red (R), green (G), and blue (B) pixels. Therefore, liquid crystal is driven by the electric field between the drain lines and the common electrode to cause light leakage. As a result, there is a fear of lowered contrast. The light leakage from an overlapping region between the drain lines and the common electrode can be prevented by forming a shielding film such as a black matrix between the pixels. In this case, however, there is another fear in that an aperture ratio cannot be improved.
On the other hand, when the technology described in Japanese Patent Application Laid-open No. 5-265045 is applied to the liquid crystal display device having the configuration described above, a drain line DL can be formed for each set of two pixels PXL, as illustrated in FIG. 6. As a result, as illustrated in FIG. 7 corresponding to a sectional view taken along the line VII-VII of FIG. 6, a region in which the drain line DL and a common electrode CT overlap each other can be reduced for each set of two pixels. The formation of the shielding film such as a black matrix BM for each region in which the drain line DL and the common electrode CT overlap each other, specifically, for each set of two pixels, enables the prevention of the light leakage from the overlapping region. With the configuration described above, however, it is difficult to greatly improve the aperture ratio. Further, the pixels adjacent to each other without the intermediation of the light-shielding film and the pixels adjacent to each other through the intermediation of the light-shielding film are periodically arranged among the three pixels, that is, RGB pixels arranged so as to be adjacent to each other. As a result, there is a fear of occurrence of unevenness in display.